The present invention relates to a semiconductor device and a method of producing the same. More particularly, it relates to techniques which are effective when applied to a resin-encapsulated semiconductor device.
A resin-encapsulated semiconductor device having a high packaging density, there is the flat packaging type of QFP (Quad Flat Package) structure. The resin-encapsulated semiconductor device of the QFP structure is so constructed that a semiconductor pellet mounted on the front surface of a tab is sealed in a resin mold. The tab is arranged so as to be enclosed with the base ends of a plurality of inner leads radiately extended in four directions. The other end sides of the inner leads are made so as to be unitary with outer leads. The tab, the inner leads and the outer leads are all formed from an identical lead frame. The lead frame is fabricated by punching or etching. The base ends of the inner leads and the corresponding external terminals (bonding pads) of the semiconductor pellet are electrically connected by pieces of bonding wire, respectively.
Regarding the resin-encapsulated semiconductor device of the QFP structure, the number of the outer leads (the number of pins) tends to increase. A resin-encapsulated semiconductor device which is being developing, is provided with 200 or more outer leads though not publicly known. The resin-encapsulated semiconductor device of the QFP structure with so many pins has its outward size enlarged and its leads thinned on account of limitation on the pitches between the leads. In particular, the inner leads concentrate toward the mounted position of the semiconductor pellet and are therefore made still finer than the outer leads. In addition, since the semiconductor pellet is reduced in size on account of a heightened density of integration, the inner leads become longer inevitably. Thinning and lengthening the leads result in increasing inductance. Among the leads, leads for power sources undergo great currents when circuits at the input and output stages of the semiconductor pellet operate at the same time. Due to the inductance components mentioned above, large currents incur fluctuations in supply voltage for use in circuits included in the semiconductor pellet and cause power source noise. For this reason, malfunctions often occur in the circuits built in the semiconductor pellet, so that the electrical reliability of the resin-encapsulated semiconductor device degrades.
A technique disclosed in U.S. Pat. No. 4,835,120 is effective for solving such a technical problem. The technique consists in furnishing a resin-encapsulated semiconductor device of QFP structure with a metal plate for a power source. The metal plate for the power source is arranged opposite the inner leads through an insulator. It is electrically connected with the external terminal of the semiconductor pellet near this external terminal by a bonding wire. Also, it is electrically connected with the end side of the power-source inner lead remote from the external terminal by welding. That is, the power-source metal plate is inserted electrically in parallel with the power-source inner lead and can lower the inductance of the latter. Accordingly, the resin-encapsulated semiconductor device of the QFP structure can reduce the power source noise owing to the metal plate for the power source.